It has long been an objective in the making of carbonless paper incorporating gelatin based microcapsules as color forming agents to deliver a high concentration of the microencapsulated color-forming components onto the paper substrate. This has been difficult to accomplish, however, due to the difficulty in obtaining a high concentration of microencapsulated internal phase (core) product in the aqueous dispersions and emulsions which are most conveniently employed as aqueous systems to deliver the microencapsulated color-forming components onto the paper substrate.
The use of aqueous delivery systems for the microencapsulated color-formers permits pumping and delivery (coating) techniques which are compatible with high speed paper coating operations.
In prior attempts to increase the concentration of the gelatin based microencapsulated color-formers, the viscosity increased to such an extent as to make the paper coating impractical and inefficient due to the adverse effects this increased viscosity had upon the pumping systems and delivery system in the paper coating operation.
Conventional aqueous microencapsulation of carbonless core color phase using gelatin as a basic microcapsule raw material has been observed in the prior art to be limited to concentrations of approximately 20 weight percent total solids as a maximum practical concentration. This was noted in U.S. Pat. No. 4,760,108 wherein it was stated at column 1, lines 40 through 52, that when making microcapsules which are obtained by the complex coacervation process making use of gelatin and an anionic electrolyte of a high molecular weight, there were several problems encountered including the difficulty in obtaining microcapsule systems having a solid content higher than 20% due to the mechanism of the coacervation process. It was further stated that these microcapsules have low productivity per unit volume and require high transportation cost and when used as a coating material for carbonless copying paper and the like, a great deal of water has to be caused to evaporate for drying the coated materials, leading to still-standing serious problems on the efficiency of coating work and energy cost.
Such prior art microencapsulation systems were generally comprised of gelatin as a basic polymer and using anionic microencapsulation polymer, such as gum arabic, polyvinyl methyl ether maleic anhydrides copolymer (PVM/MA), carboxy-methyl cellulose, and combinations thereof with other polymers.
An alternate approach, such as in U.S. Pat. No. 4,087,376 issued to Peter L. Foris et al, is directed to a process for preparing non-gelatin microcapsules, en masse, by an in situ polymerization reaction to yield capsule wall material. The polymerization includes a reaction between urea and formaldehyde or polycondensation of monomeric or low molecular weight polymers of dimethylol urea or methylated dimethylol urea in an aqueous vehicle with the reaction being conducted in the presence of a negatively-charged, carboxyl-substituted, linear aliphatic hydrocarbon polyelectrolyte material dissolved in the vehicle. Liquid-liquid phase separation is accomplished and maintained by increase in the molecular weight of the resulting condensation polymer without further dilution of the manufacturing vehicle. The negatively-charged polyelectrolyte material is required and has an apparent effect of controlling or modifying the polymerization reaction. The Foris et al microencapsulation process is stated by the patentees to permit the manufacture of microcapsules in concentrations of microcapsule to microcapsule manufacturing vehicle higher than previously possible.
U.S. Pat. No. 4,760,108 to Makoto Asano et al is directed to microcapsule-containing, water-base coating formulation comprising as essential components (a) microcapsules making use of a synthetic resin as a wall-forming material and (b) a reaction product obtained by polymerizing at least one water-soluble vinyl monomer (B) in the presence of a high polymer latex (A) having a glass transition point of 60 degrees C. or lower. The latex (A) and vinyl monomer (B) are used at a solid weight ratio of 3.97-90:10. The water-base coating formulations are stated to provide a microcapsule-coated layer having significantly-improved pressure resistance and frictional stability without need for a stilt. The water-base coating formulation is stated to be capable of being applied at a high speed, thereby making a significant improvement to the productivity of carbonless copying and/or recording paper.
U.S. Pat. No. 4,205,060 issued to H. G. Monsimer et al is directed to the preparation of microcapsules appearing to consist of strings of spheres with the individual spheres in the string being about 20 to 30 microns in diameter. Methyl cellulose or ethyl cellulose can be employed in conjunction with a polyvinyl methyl ether/maleic anhydride (PVM/MA) polymer such as "Gantrez AN-169" to produce these strings of spheres with the internal phase being various medicaments. The Monsimer et al microencapsulation procedure is a non-gelatin system.
U.S. Pat. No. 4,808,408 issued to Richard W. Baker et al is directed to the formation of microcapsules using gelatin and gum arabic hardened with a combination of formaldehyde and glutaraldehyde. In Example 5 of the Baker et al patent, it is stated that a copolymer of methyl vinyl ether and maleic anhydride ("Gantrez AN-119") functions as a pharmaceutical vehicle, e.g., to produce a resulting cream for application to human skin. The formulation of Example 5 contains 91 parts by eight of microcapsules with 9 parts by weight of the "Gantrez AN-119" solution as a pharmaceutical vehicle. The microcapsules have microcapsular material made from gelatin and gum arabic hardened with a combination of glutaraldehyde and formaldehyde.
U.S. Pat. No. 4,082,688 issued to Setsuya Egawa et al teaches using polyvinyl methyl ether/maleic copolymer and gum arabic hardened with aqueous glutaraldehyde to obtain a microcapsular system having more than 95%, by number, of the microcapsules being mononucleic. These mononucleic microcapsules are stated to be of single emulsified droplets. These microcapsules were then coated onto test paper and color developed when used in conjunction with a sensitized receiving sheet after marking same.
U.S. Pat. No. 3,872,024 issued to Georg Horger is directed to a process for microencapsulation using gelatin in liquid-liquid phase separation utilizing certain inorganic polymeric materials as phase-separation-inducing materials. The Horger microencapsulation process employs simple coacervation of gelatin and employs inorganic polymer materials, e.g., sodium polymetaphosphates as phase separation inducing agents. Example 1 of Horger illustrates use of sodium hexametaphosphate (Calgon) as the phase separation inducing agent in microencapsulation of oily core materials, e.g., coloring agents using a gelatin microcapsule raw material which can be hardened later using an aldehyde(s). The microcapsules produced by the Horger patent process are characteristically large microcapsules, e.g., hundreds of microns in diameter.
U.S. Pat. No. 4,105,823 issued to David John Hasler et al is directed to a method of microencapsulating finely divided particulate material, such as minute droplets of a water-immiscible liquid, to produce microcapsules in which the particulate core material is surrounded by polymeric shells. This method comprises the steps of forming a dispersion of the particular material in an aqueous medium containing a water-soluble urea formaldehyde precondensate, a water-soluble melamine formaldehyde precondensate and a water-soluble polymer which is capable of being cross-linked by said precondensates, and condensing said precondensates by acid catalysis with resultant cross-linking of the polymer about the particulate material so as to form the polymeric shells. The polymer may be a cellulose derivative, starch, a starch derivative, a polyacid, a polyester, a polyanhydride copolymer, a polyacrylamide or a polyacrylamide copolymer, and is preferably an acrylamide/acrylic acid copolymer.